1. Field of the Invention
The present invention relates to a system and method for digital broadband frequency measurement.
2. Description of Related Art
The conventional instantaneous frequency measurement (IFM) receiver is a radio frequency (RF) receiver used primarily in electronic warfare. The IFM measures the frequency of pole signals radiated from a hostile radar and measures the frequencies of incoming RF signals utilizing interferometric techniques by detecting the phase shift magnitudes produced in multiple calibrated delay lines. During normal operation, the received RF signal may be divided and simultaneously introduced into a non-delayed path and a delayed line of known length. The phase differences between the delayed and non-delayed receiver paths are functions of the input signal frequency that include the conversion of the phase different signals and provide signals whose amplitudes are related to the phase delay. The signals typically take the form of sin wt or cos wt where the w is the angular frequency of the L-processed input signal. The sin wt/cos wt signals are delivered to the encoding network which makes the amplitude comparisons of the signals, determines the numeric value of w and generates the digital frequency descriptive word.
The IMF typically splits the input signal into two or more constituent signals by the use of a power divider. One constituent signal is applied to the referenced delay line and the remaining constituent signals are applied to the differential delay lines. Delay lines are applied to the separate channels of a N-channel phase measurement receiver where the difference in delay between the referenced delay line and the differential delay lines causes a frequency dependent phase shift, which is measured by the N-channel phase measurement receiver. The frequency of the input signal is determined from this phase shift by the phase of frequency decoder. The IFM systems existing in the prior art typically use RF components and the delayed lines as described above to accomplish a cascaded set of RF frequency discriminators, which are combined to create a frequency estimate of the largest signal entering the device to an accuracy of about one MGhz. IFM systems typically operate with input bandwidths of several Ghz and an input signal pulses to under 100 nanoseconds.
U.S. Pat. No. 6,411,076 to Rudish (Rudish) relates to an instantaneous frequency measurement receiver that receives a signal from a target and determines the frequency of the signal. The IFM receiver of Rudish uses a method that seeks to minimize the total number of delay lines required to achieve a given accuracy and frequency measurement. The method of Rudish attempts to provide an IFM receiver that minimizes the quantity of delay lines and thus provides a less costly and complex IFM receiver as opposed to the prior art.
U.S. Pat. No. 5,235,287 to Sanderson, et al. (Sanderson) relates to an IFM receiver that attempts to provide a bandwidth improvement through phase shift sampling of real signals. Sanderson discusses a method that allegedly doubles the unambiguous bandwidth of a frequency measurement receiver and measures frequencies over a wide range at a very high sampling frequency. Sanderson, as other IFMs known in the prior art, uses original and delayed signals that are sampled simultaneously. Sanderson discusses the extension of the frequency range by the implementation of an analog to digital converter that receives the output of the IFM.
Although IFM receiver technology is well known and fairly effective, the major disadvantage of the prior art relates to the size, weight and costs associated with the production of the receivers. The IFM receivers used in the prior art usually require analog delay lines that can be up to ten to twenty feet long and therefore increase the size and weight associated with the devices. The size of cable may be limited because it cannot be infinitesimally small since it needs to operate at several GHz frequencies with reasonably small losses. The IFM receivers are typically custom designed and built in small quantities and, therefore, require a significant amount of labor and calibration associated with them.